diff --git a/core/fxcodec/codec/fx_codec_fax.cpp b/core/fxcodec/codec/fx_codec_fax.cpp
index 336c35b..c121478 100644
--- a/core/fxcodec/codec/fx_codec_fax.cpp
+++ b/core/fxcodec/codec/fx_codec_fax.cpp
@@ -390,6 +390,10 @@
     if (a1 >= columns)
       return TRUE;
 
+    // The position of picture element must be monotonic increasing.
+    if (a0 >= a1)
+      return FALSE;
+
     a0 = a1;
     a0color = !a0color;
   }